The present invention relates to a method and apparatus for monitoring the positions and operating sequence of turbine governor valves, particularly in a turbine-generator system, and determining malfunctions or inefficiencies therein.
The flow of driving steam to a turbine is generally regulated by governor valves via which steam is delivered from a high-pressure steam source to the inlet nozzles of a high-pressure turbine stage. Since the turbine stage generally has a plurality of nozzles distributed around its circumference, a separate governor valve is provided for supplying steam to each nozzle. Depending on the operating requirements of the particular turbine system, all valves can be controlled to operate in unison or in a certain sequence.
Each valve can operate between a fully closed state and a fully open state. When a governor valve which is initially fully closed is to begin passing steam, it is usually caused to jump immediately to approximately 7 percent of its total displacement, or lift. The valve is then said to be at its crack point. There is usually a small amount of play in the valve plug and the valve stem must move through a small distance before the valve "cracks" and begins to pass steam.
Then, as the valve stem continues to be displaced in the opening direction, flow through the valve increases approximately linearly until a further point is reached, known as the knee point. During this portion of valve displacement, the valve is controlling, or modulating, steam flow. The knee point, which usually occurs after the valve has been displaced over 30-40 percent of its total path, corresponds to the establishment of nearly full flow through the valve. A valve is usually controlled so that upon reaching its knee point, it is moved to the open end of its displacement path.
A regular increase in flow occurs as the valve closing element is moved from its crack point to its knee point and the knee point represents the point at which an abrupt increase occurs in the slope of the displacement vs. flow curve.
If the desired control of flow of steam to the high-pressure turbine stage requires that a governor valve operate in the region between its crack point and knee point, steam flowing through the partly opened valve is being throttled by the valve, which has an adverse effect on the efficiency of the high-pressure turbine and on the heat rate of the power plant.
Turbine-generator load is a function of a steam flow which, in turn, is a function of turbine governor valve position. Turbine governor valves can be operated in two modes: single valve mode (in which all governor valves move in unison) and sequential valve mode (in which the governor valves operate individually in a preset sequence). At loads less than full load, sequential valve mode operation is more efficient than single valve mode operation. In sequential valve mode, the most efficient turbine operation is achieved at a valve point. A valve point is defined as the point at which a governor valve is open as much as possible before the next valve in sequence begins to open. There are several distinct valve points depending on the number of governor valves. Operation between valve points is inefficient because of steam throttling losses through partially open governor valves. This is sometimes unavoidable due to utility dispatch load requirements. This is called operating on a valve loop because of the "loop" in the heat rate curve between valve points. Operating on a valve loop can cause heat rate losses of up to 50 Btu/kWh.
The pressure conditions associated with sequential valve operation are depicted in the curve of FIG. 1, which represents the relation between pressure and flow or load percent with respect to each valve. This diagram relates to a system employing six governor valves, two of which operate in unison and the other four of which operates sequentially for supplying steam to the first stage, or high-pressure stage, of a multistage turbine. Steam is supplied to all of the governor valves via throttle valves whose inlet pressures remain essentially constant, as shown by the horizontal broken line in FIG. 1. The pressure with which steam is supplied to the first turbine stage is also shown.
Curve 10 represents the outlet pressure of the first two valves which are to be opened as the turbine begins operation in the lower point of its load range. These valves can be controlled from their fully closed condition, corresponding to turbine shutdown, to their fully opened condition, corresponding to knee point 12. When the first two valves reach their knee point, at which they are supporting nearly their full flow, and all of the other valves remain essentially closed, the turbine is operating at its lowest valve point.
If the turbine is to operate at a higher load level, the next valve in the sequence is opened; the variation in outlet pressure of that valve as it opens from its crack point 14 to its knee point 16 is represented by curve 18. When the valves represented by curves 10 and 18 are passing their full flow, the turbine is operating at the next valve point.
Correspondingly, the operating the level of the turbine can be increased by opening one or more of the next three valves in sequence, for which the pressure variations between crack points 20, 22 and 24, and knee points 26, 28 and 30 are represented by curves 32, 34 and 36, respectively.
The outlet pressures of the valves which are already fully open are represented generally by the common curve 37, which represents a pressure differing from the throttle valve inlet pressure by an amount 38 constituting the pressure drop across the throttle valve and the open governor valves for each load value. The vertical distance between the curve representing the outlet pressure of each governor valve and the first stage pressure corresponds essentially to the pressure drop across the associated nozzle which is supplied with steam via the valve.
In systems employing sequential valve actuation, it is desirable that a certain overlap 39 exist between the load point associated with the knee point of one valve and the load point associated with the crack point of that valve which is to open next in the sequence if the load is increasing, or which closed previously in the sequence if the load is decreasing. If such overlap does not exist, then a "flat spot" will appear in the load response, and this can be source of operating instability. On the other hand, an excessive overlap creates an inefficient operating condition.
In addition, when sequential valve operation is employed, it is important that the valves be operated in a sequence such that steam is supplied to the turbine stage over only a single contiguous portion of its nozzle circumference. If steam were supplied at two angularly separated portions of the nozzle circumference, with no steam being supplied between those portions, this would produce a condition known as double shock which can place severe stresses on the turbine stage and may lead to blade failure in a short period of time. While such condition should not occur if the governor valves are operating properly, it could occur due to breakage of a governor valve stem.
It is known to monitor turbine governor valve alignment on the basis of measurements of throttle pressure, throttle temperature and first stage pressure. Such an approach is described, for example, in a publication entitled EPRI first use, document FS5429B/E, published in December 1985.
It is also known to employ governor valve outlet pressure readings to set governor valve points during field tests.